I'll occasionally wear a long sleeve Champion t-shirt in their thin, lightweight wicking fabric, white or hi-viz yellow or orange, under my cycling jersey. Works well on hot sunny days that aren't too humid.

The Champion tees use a very thin material that isn't substantial enough to support pockets, so it's significantly thinner and lighter than most cycling jerseys. Soft and comfortable, breathes well. Doesn't cost much. These aren't form fitting, however. Champion also makes compression tees, long and short sleeve. Haven't tried 'em yet.

To me it's more comfortable and less hassle than using sun screen. I still use sunscreen on my face and neck, and exposed areas on my legs. But it feels sticky and my legs in particular get grimy after a long summer ride. The long sleeve tees are a good compromise for sun protection and comfort.

I really like the Pearl Izumi Thermal baselayers year round, except for the most humid summer days. And I've worn it under dress shirts for summer visits to museums, theater, etc. But they're pricey and I have only a sleeveless baselayer. I might try a Champion compression long sleeve base layer to compare with the Pearl Izumi.